This study is aimed at the evaluation of THz gain properties in an optically pumped NH3 gas. NH3 molecules undergo rotational-vibrational excitation by mid-infrared (MIR) optical pumping provided by a MIR quantum cascade laser (QCL) which enables precise tuning to the NH3 infrared transition around 10.3 μm. Pure inversion transitions, (J = 3, K = 3) at 1.073 THz and (J = 4, K = 4) at 1.083 THz were selected. The THz measurements were performed using a THz frequency multiplier chain. The results show line profiles with and without optical pumping at different NH3 pressures, and with different MIR tuning. The highest gain at room temperature under the best conditions obtained during single pass on the (3,3) line was 10.1 dB×m−1 at 26 μbar with a pumping power of 40 mW. The (4,4) line showed lower gain of 6.4 dB×m−1 at 34 μbar with a pumping power of 62 mW. To our knowledge these THz gains are the highest measured in a continuous-wave MIR pumped gas.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz photonics is one of the newer fields in electromagnetic research. The terahertz band lies between microwave and infrared frequencies thanks to which it possesses unique properties. It has promising applications in spectroscopy, nondestructive testing, wireless communications, imaging, detection of hazardous substances and much more [1,2]. Despite some progress, THz photonics lacks a reliable, compact and powerful THz source. Thermal THz sources can be used in applications where incoherent higher THz frequencies are sufficient (Fourier Transform Infrared Spectroscopy), however, their power is very low and therefore they require cryogenic detectors. Optoelectronics  and electronic harmonic generation can be used to synthesize lower THz frequencies, but have a limited efficiency close to 1 THz and above. Another method uses nonlinear crystals (parametric effect, difference frequency generation) but generally needs very high power continuous or pulsed lasers. Direct THz generation - lasers, can be divided in two categories: optically pumped molecular lasers (OPMLs) and semiconductor lasers (terahertz quantum cascade lasers and p-Ge laser). Terahertz quantum cascade lasers (THz QCLs) are relatively new sources demonstrated in 2002 . They can be small with reasonable output power and some tunability, however they suffer from decreasing power with temperature and still require cryogenic cooling . THz OPMLs were demonstrated in 1970 and the original design can still be found in commercial THz lasers [6, 7]. They are powerful but they are bulky, not efficient and have limited tunability. Optical pumping in the mid infrared (MIR) region can be seen as a drawback if a CO2 (or N2O) discharge laser is used. The tunability of these kind of lasers is also limited due to the required coincidence of the line provided by the pumping laser with the MIR absorption line of the THz active molecular gas. This condition limits also drastically the choice of the molecule and of the transition. Thanks to the development of new MIR quantum cascade lasers (MIR QCLs) we may overcome some disadvantages of the CO2 pump laser. MIR QCLs are small, have a high efficiency and can be precisely tuned which allows to excite lines of molecular gases that cannot be accessed by a conventional CO2 pump. In addition, commercially available MIR QCLs are operated around room temperature.
A terahertz molecular laser optically pumped by a MIR QCL based on ammonia (NH3) was demonstrated recently  and offers many new possibilities. It is essential to know the most important parameters  and the behavior of the lasing lines if one wants to optimize the design of a laser. In this paper a study of the THz gain properties of two selected optically-pumped NH3 lasing lines near 1 THz at room temperature is presented. Compared to our previous results [10,11] important information about the gain, the lineshape and the pressure dependence are obtained.
2. Theoretical background
Ammonia (NH3) is a well known molecule that was used in some lasing experiments starting with the first demonstration of stimulated emission and the construction of the first maser . High dipole moment (1.4 D), high rotational constant and fast relaxation rate makes it a good choice for THz OPMLs . Absorption of NH3 in the infrared spectral range around 960 cm−1 is associated with the ν2 normal modes which correspond to a symmetrical deformation of N-H interatomic bonds, also called umbrella mode. The energy levels are also split due to the tunneling of nitrogen atom through the barrier created by the three hydrogen atoms . These levels are split into the lower energy symmetric (s) and the higher energy antisymmetric (a) wavefunctions (Fig. 1). NH3 is also a symmetric top molecule whose rotational state is expressed by the quantum numbers (J, K) where J is the total rotational angular momentum and K is the projection of the angular momentum to the symmetry axis. The energy differences between the a and s levels for the vibrational ground state ν2 = 0 lie in the microwave region. The first maser was demonstrated by inversion of population between a and s levels of the (3,3) line at a frequency close to 24 GHz . In the first vibrational excited state ν2 = 1 the difference between a and s increases and the splitting becomes wider with a frequency around 1 THz. The selection rules for rotational inversion transitions are Δν = ΔK = 0, ΔJ = 0, ±1, a ← s and s ← a. The selection rules for parallel vibrational transitions like ν2 are Δν = ±1, ΔK = 0, ΔJ = 0, ±1 and a ← s, s ← a .
In this article we study excited molecules optically pumped using Q-branch transitions Δν2 = 1, ΔJ = ΔK = 0, a ← s and population inversion on pure inversion transitions ΔJ = ΔK = 0, s ← a (Fig. 1). The molecules relax by collisions with other molecules or with cavity walls. For the experiment we selected two lines: (1) pumping at 967.3463 cm−1 (saQ(3,3) transition) and probing around 1073049.6 MHz (ν2 = 1 asQ(3,3)), (2) pumping at 966.8147 cm−1 (saQ(4,4) transition) and probing around 1082592.4 MHz (ν2 = 1 asQ(4,4)) .
The experimental setup is shown in Fig. 2. The THz probe beam was generated by a frequency multiplier chain (Virginia Diodes, Inc.) equipped with WR-0.65x3x3  stages (9 times frequency increase). The input signal for the multiplier was synthesized by a high precision microwave generator (Agilent) with an amplitude modulation used for a lock-in detection. An InSb hot electron bolometer (QMC Instruments Ltd.) cooled to 4 K was employed for the detection of the THz radiation. The NH3 gas flowed through a gas cell made from a 50 cm long copper tube with 10 mm inner diameter. The dimensions of the tube were chosen based on a compromise between gain, mode nonuniformity and waveguiding effect . The gas cell was terminated by THz and MIR transparent high resistivity Si window inclined at the Brewster angle in order to avoid any reflection in the cell. The cell was tested using a helium leak detector with no leaks detected. The NH3 gas was injected at constant flow from a cylinder through a needle valve, which was used to control the desired pressure, into the gas cell and pumped by a turbomolecular pump. A small flow of NH3 during measurement prevents from a cell walls contamination and helps to stabilise the pressure. The temperature of NH3 was approximately 20°C. MIR optical pumping was provided by a distributed feedback MIR quantum cascade laser (QCL) with a central wavelength around 10.3 μm (AdTech) and a power up to 100 mW. A polymer lens (Zeonex) was used to focus the THz beam and a 200 mm focal length ZnSe lens was used to focus the MIR beam into the gas cell. The THz beam was collected by gold parabolic mirrors and focused into the bolometer. A step attenuator (Lasnix) was used to reduce the power of the MIR beam when needed. A semi-insulating SiC wafer which reflects MIR and is transparent for the THz beam was used as a beam combiner. Polarization (p) and propagation directions of the THz and MIR beams entering the gas cell were the same. The spot size of the MIR beam at the entrance of the cell was approximately 2 mm diverging up to 10 mm at the end of the cell.
First the THz spectra of the two lines were recorded without MIR pumping (QCL off). The NH3 pressure was stabilized and the multiplication chain was swept with a frequency step of 108 kHz over a sufficient range around the line center. Figure 3 shows profiles of the NH3 (3,3) (left) and (4,4) (right) lines at different pressures. The output power of the multiplication chain measured by a pyroelectric detector was 3.2 μW at 1.073 THz and 6.8 μW at 1.082 THz. The Gaussian shape of the absorption lines at such low pressure is caused dominantly by Doppler broadening . The line center is slightly shifted from its theoretical value with increasing pressure by the collisional shift .
Figure 4 shows (3,3) (left) and (4,4) (right) lines with optical pumping by the MIR QCL at a NH3 optimal pressure of 26 μbar and 34 μbar respectively (for determination of the optimal pressure, see Fig. 5 right). The lasing linewidth of the MIR QCL is narrower than the Doppler broadening of the MIR transitions of NH3.
The QCL pumps only a category of molecules with a particular velocity. The position of the THz gain peak then depends on the MIR pumping frequency and as a result the THz gain bandwidth is narrower than the Doppler linewidth. The maximum gain is obtained at the center of the THz lines when the QCL is tuned precisely to the center of the MIR transitions. The tuning curves (inserts in Fig. 4) of the THz peak gain frequency vs. MIR frequency have a linear trend with a slope equal to the ratio of the THz and the MIR transitions (27 for (3,3) and 26.8 for (4,4)). The power of the MIR beam measured by a thermopile at the exit of the evacuated cell was close to 40 mW for the (3,3) line and 62 mW for the (4,4) line with an estimated error of 5 %. The absorption of the pump power at 26 μbar in the 50 cm long cell was 53% for the (3,3) line and 26% for the (4,4) at 34 μbar. The linewidth of the MIR QCL is generally broadened by small current  and temperature fluctuations. Here, it is estimated to be 35 MHz and can be further improved by various methods .
Figure 5 (left) shows the transmittance as a function of the MIR pumping power. The transparency when the stimulated emission is equal to the absorption is about 1 mW for both lines. The gain dependence on the pump power is linear up to approximately 13 mW where probably depletion of the ground states starts, probably due to the slow vibrational relaxation of ν2 = 1(s) states after stimulated emission . The slope of the linear part of the gain for the (3,3) line is 0.08 mW−1 and 0.05 mW−1 for the (4,4) line. The effectiveness may be increased by adding different gas to help with the relaxation to the ground state .
The optimal NH3 pressure is an important parameter that heavily affects gas laser performance . Figure 5 (right) shows the THz gain as a function of the pressure. The gain increases linearly with the pressure due to the higher number of molecules in the volume up to a certain point where collisions between molecules and/or cavity walls cause increased nonradiative relaxation  and exponentially decrease of population inversion. The optimal pressure is between 20 – 30 μbar and 25 – 45 μbar for (3,3) and (4,4) line respectively.
The final NH3 gain and pump absorption per unit length were calculated from the Beer-Lambert law for the single pass through the active medium : I(d) = I0 × exp(αd), where I0 and I(d) are the intensities at the input and at the output distance d (cell length), and α represents the absorption coefficient for negative values and gain coefficient for positive values. The maximum gain during the experiment for the (3,3) line at the best conditions (26 μbar, 40 mW pump power, 50 cm long path) was 3.2 which equals 2.3 m−1 or 10.1 dB×m−1. The maximum gain for the (4,4) line at the best conditions (34 μbar, 62 mW pump power, 50 cm long path) was 2.1 which equals 1.4 m−1 or 6.4 dB×m−1. A signal to noise ratio of the gain measurement was estimated to be higher than 20 dB. These values are to our knowledge the highest measured in a continuous wave MIR pumped gas in the THz range [9,26,27].
The THz optically pumped molecular lasers (THz OPMLs) are widely applied as sources of coherent THz radiation. Progress in tunable MIR quantum cascade lasers (QCLs) allows the replacement of normally used CO2 laser MIR pump, which gives us the opportunity to excite transitions that were not accessible due to the required coincidence with CO2 lines. The precise tuning of the MIR QCL allows a selective pumping and gives access to a number of new THz lasing lines in various molecular gases. NH3 based THz OPML pumped by MIR QCL was successfully demonstrated  and opened up new possibilities in the field of THz lasers. In this article we presented measurements of important parameters of optically pumped NH3. During optical pumping molecules undergo roto-vibrational inversion excitation to the vibrational level ν2 = 1 followed by stimulated emission on pure inversion transitions and nonradiative relaxation to the ground vibrational level. Two lines were selected at 1.073 and 1.083 THz. Measurements were performed at different pumping power, frequency and pressure values. Measured gains up to 10.1 dB×m−1 and up to 6.4 dB×m−1 were obtained during one-way copropagation for the (3,3) and (4,4) lines respectively. We attribute the high gain measured to the strong inversion transition chosen and to the fact that the QCL allows perfect resonant pumping. It is possible to achieve even higher gain by improving conditions, e. g. matching the THz probing beam with a preferred waveguiding mode of the cell [10, 17]. Obtained parameters will help a better understanding of the lasing gas behavior and may serve as an information for improving performance of the existing NH3 THz OPML.
Ministry of Education, Youth and Sports of Czech Republic (SP2018/98, LQ1602), Agence Nationale de la Recherche (ANR-16-CE30-0020 HEROES), Czech Science Foundation (18-22102S), European Regional Development Fund (CZ.02.1.01/0.0/0.0/16_013/0001791).
This work was done in the framework of the PIA Equipex ExCELSiOR project.
The authors declare that there are no conflicts of interest related to this article.
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